1. Field of the Invention
The present invention relates to a sensor and a method for fabricating the same, and more particularly, to an image sensor and a method for fabricating the same.
2. Background of the Related Art
FIG. 1 is a layout illustrating a conventional solid state image sensor, and FIG. 2 is a sectional view illustrating a conventional solid state image sensor. FIGS. 3A to 3F are sectional views illustrating a method for fabricating a conventional solid state image sensor of FIGS. 1 and 2.
As shown in FIG. 1, a solid state image sensor includes a plurality of photodiode regions 300, vertical charge transfer regions 400, a horizontal charge transfer region 500, and a sense amplifier 600. The plurality of photodiode regions 300 converts a signal of light to an electrical image transfer signal. Each of the vertical charge transfer region 400 transfers the image charge formed by the photodiodes 300 in a vertical direction. The horizontal charge transfer region 500 transfers the image charge transferred in a vertical direction by the vertical charge transfer region 400 to a horizontal direction. The sense amplifier 600 senses the image signal charge transferred in the horizontal direction by the horizontal charge transfer region 500.
As shown in FIG. 2, a conventional solid state image sensor includes a p-type well 12 formed in a surface of an n-type semiconductor substrate 11 in which a photoelectric conversion region is defined. A photodiode is formed of a PD-N region 13 and a PD-P region in a surface of the p-type well 12 in the photoelectric conversion region, for converting a signal of light to an electrical signal. A vertical charge transfer region 14 is formed in the surface of the p-type well 12 in which the photodiode is not formed, and a channel stop layer 15 is formed in the surface of the p-type well 12 around the photodiode except for a portion between one side of the photodiode and the vertical charge transfer region 14.
A gate insulating film 16 is formed on the semiconductor substrate 11 including the vertical charge transfer region 14 and the photodiode, and a transfer gate 17 is formed on the gate insulating film 16 except for the photodiode. A first insulating film 18 is formed on a surface of the transfer gate 17, and a second insulating film 20 is formed on the gate insulating film 16 including the first insulating film 18. A light-shielding layer 21 is formed on the second insulating film 20 except for the photodiode, and a third insulating film 22 is formed on the light-shielding layer 21 including the second insulating film 20.
A conventional method for fabricating a solid state image sensor will be described with reference to FIGS. 3A to 3F. As shown in FIG. 3a, a p-type impurity ion is selectively implanted into a predetermined region in a surface of an n-type semiconductor substrate 11 in which a photoelectric conversion region is defined. A p-type well 12 is then formed by a drive-in diffusion process.
As shown in FIG. 3B, an n-type impurity ion is implanted into a surface of the p-type well 12 in the photoelectric conversion region. A PD-N region 13 is then formed by drive-in diffusion process. Subsequently, a heavily doped n-type impurity ion is implanted into the surface of the p-type well 12 in which the PD-N region 13 is not formed. A vertical charge transfer region 14 is then formed by a drive-in diffusion process.
As shown in FIG. 3C, a heavily doped p-type impurity ion of energy lower than that to form the PD-region 13 is implanted into the surface of the p-type well 12 around the PD-N region 13 except for a portion between one side of the PD-N region 13 and the vertical charge transfer region 14. A channel stop layer 15 is then formed by a drive-in diffusion process. A gate insulating film 16 is formed on an entire surface of the semiconductor substrate 11 in which the vertical charge transfer region 14 and the channel stop layer 15 are formed.
As shown in FIG. 3D, a polysilicon and a first photoresist are sequentially formed on the gate insulating film 16. The first photoresist is selectively patterned by exposure and developing processes to remain over the vertical charge transfer region 14. Subsequently, the polysilicon is selectively etched using the patterned first photoresist as a mask to form a transfer gate 17. The first photoresist is then removed. A first insulating film 18 is grown on a surface of the transfer gate 17 by a thermal oxidation process.
As shown in FIG. 3E, the heavily doped p-type impurity ion of energy lower than that to form the channel stop layer 15 is implanted into the surface of the p-type well 12 in the photoelectric conversion region. A PD-P region 19 is then formed with a thin thickness by a drive-in diffusion process. Thus, a photodiode with the PD-N region 13 and the PD-P region 19 is formed.
A second insulating film 20 and a light-shielding layer 21 are sequentially formed on the entire surface including the first insulating film 18. As shown in FIG. 3F, a second photoresist is deposited on the light-shielding layer 21 and then the second photoresist is selectively patterned by exposure and developing processes, so that the second photoresist over the photodiode is removed.
Subsequently, the light-shielding layer 21 is selectively etched using the second patterned photoresist as a mask. The second photoresist is then removed. A third insulating film 22 is formed on the entire surface including the selectively etched light-shielding layer 21.
In the conventional solid state image sensor, a signal charge stored in the photodiode is transferred to the vertical charge transfer region 14 by a clock signal applied to the transfer gate 17 and then is moved in the vertical direction. A hole or the positive charge is then removed in the PD-P region 19.
The PD-N region 13 is originally floating. However, if a high clock signal is applied to the transfer gate 17, the signal charge in the PD-N region 13 is transferred to the vertical charge transfer region 14 and electrons are thus lost. As a result, the PD-N region 13 is pinched off. In this state, if the signal charge occurs again, the potential of the PD-N region 13 ascends, but fails to ascend more than Vsdl (saddle potential) of the p-type well 12.
The vertical charge transfer region 14 has high potential because a voltage of the transfer gate 17 is added to the pinched off potential of the PD-N region 13. At this time, a high voltage is applied to the transfer gate 17 because the device is operated by deep depletion mode.
However, the conventional solid state image sensor and the method for fabricating the same have several problems and disadvantages. If a surface of the photodiode is damaged or contaminated by a heavy metal in the course of the process steps, a noise charge occurs, and the noise charge flows to the photodiode. In addition, since the gate insulating film is formed on the semiconductor substrate including the photodiode, the incident light directly flows to the vertical charge transfer region through the gate insulating film, thereby causing a Smear phenomenon.